Switches have undergone the same evolution as other areas of technology, from mechanical to electrical devices. Efforts have also been made to design switches which employ optical means, such as light being transmitted from a transmitter to a receiver. The early electric eye door opening devices are examples of basic optical switch design.
As technology has progressed, many functions which have in the past been performed electrically are now being performed using fiber optics. Telephone signal transmission is one area where fiber optical devices are enjoying great success and adoption. Fiber optical devices are admirably suited for stable transmission of light over remarkably long distances.
Fulenwider et al U.S. Pat. No. 4,071,753 discloses a transducer which converts acoustical energy directly into optical energy. This design contemplates a device in which modulated optical power is transmitted from a transducer to provide information in the form of the resulting modulation of the output power. Schuck U.S. Pat. No. 4,148,558 also uses fiber optics to convey information in systems where an optical by-pass relay is interposed for the insertion of a utilization device. Both patents are examples of the use of optical fibers which transmit light from one point to another.
It is also known to modify the nature of fiber optic transmission. U.S. Pat. No. 4,531,811 to Hicks describes a device in which a polarizer is inserted between an input fiber and an output fiber. It is stated that sheet polarizers are normally quite disadvantageous, so that polarizers which are fiber or fiber-like are used. Advantages in transmission efficiencies and in reduced loss are described.
Fiber optical switching devices are described in Quinlan U.S. Pat. No. 5,056,884, in which fiber optics form part of a switch which is used for sensing the passage of a vehicle over a treadle. Light is passed along a fiber optic cable through a region which is subjected to force from the weight of a vehicle passing over the designated area of the device where the cable is located. Quinlan describes the effect of deformation of the fiber during transmission of light by the force of the vehicle or the like, where light passing down the fiber appears virtually to be switched off by pressure applied to the device as constructed. Quinlan discloses that the movement required to give attenuation to this light is only 0.04 mm. It is noted that the fibers can be squeezed a further 0.1 mm before it is over stressed.
This does point out the most significant drawback of fiber optical devices generally, which is the inability of fiber optical materials to flex or bend without incurring significant damage to the fibers. Fibers can be assembled into a variety of shapes, using proper care of the fiber, and damage to the fiber optical properties are minimal. However, any repetitive flexing such as in a switching system of the type described in Quinlan, for example, presents a real risk of damage to the fibers. When, as in Quinlan, movement of only 0.04 mm is adequate to produce the result desired, a safety factor of 2.5 times that movement, or 0.1 mm may be appropriate for large devices where vehicle weight activates the system. For smaller devices, limitations on movement to prevent damage to the fiber optics have prevented the adoption of such fiber optical systems to any great extent. Even though fiber optical cables are capable of transmitting light over great lengths, switching and other operations which require movement of the cable have not meet with any success.
One such method to overcome the deficiencies of fiber optical switching designs is shown in Jaeger et al U.S. Pat. No. 4,701,614 where an optical fiber is surrounded by an two coating layers concentrically clan to and coaxially oriented with the optical fiber. The first layer is a relatively hard optically lossy material which absorbs light radiating outwardly from the fiber. The second coating layer is comprised of a compliant material which imparts micro-bends onto the optical fiber to cause change from optical propagating modes to nonpropagating modes. No dimensional changes are set forth in Jaeger et al, but it is clear that the second coating layer is intended to take most of the flexing force in order to protect the optical fiber itself.
At this point in time, there really have not been any successful suggestions to make optical switches from optical fibers. The need for physical movement of the fiber has prevented such use, and most proposals involve mechanical or electricto-optical devices to act as a switch at some junction of the fiber.
Another type of optical fiber has come into use in some instances, known as polymer optical fibers. Haese et al U.S. Pat. No. 4,915,473 discloses a pressure sensing device in which a polymer optical fiber is employed. This fiber includes a core formed from a flexible thermoplastic aliphatic segmented polyurethane. It is noted that this particular material provides for high flexibility, thus making it more rugged and durable than other optical fiber based pressure sensors. In Haese et al, the polymer optical fiber is subjected to pressure which is said to be much more than traditional optical fibers can take without damage. The intensity of light transmitted varies inversely with the pressure applied. It is also noted that all of the variations shown in Haese et al are designs in which the fiber is linear in alignment and the force applied is normal to the axis of the fiber.
One additional design which has attempted to employ a pressure sensitive sensor is described in Ishiharada et al U.S. Pat. No. 4,830,461. The particular cable is described as having excellent heat resistance and impact resistance. The specific problems of fiber optical systems are set forth, wherein there is an unresolvable conflict between insensitivity of glass fibers to low pressure and the deformation (and resulting damage) when high pressure is applied. For glass fiber optical fibers, as noted previously, there is no way to resolve this conflict.
Ishiharada et al describes an optical wave guide which is constructed with a core of an elastic material and a clad, with the core being made from synthetic rubber. A variety of suggested uses are described in which the pressure-sensitive sensor generates a signal which is produced by the interruption of light caused by the pressure as it is applied. The patent suggests that one major improvement is that plastic deformation of the wave guide does not cause the peeling of the light emitting and receiving from the optical wave guide portion of the cable. Apparently, because the cable is elastomeric, within limits at least the cable is stretched to keep the transducer components from pulling away from the cable as it is deformed by the pressure. It is not clear if the light transmission is due to the stretching or due to the pressure on the cable. Elastic deformation is said to produce emission of light toward the outside and also produces scattering in all directions, so that some light (but not enough to activate the light receiving sensor) is transmitted back to the transmitting end as well as toward the receiving end. In any event, Ishiharada et al does not suggest that a compact and sturdy switch can be provided which can be acted upon without direct contact by the force on the cable to cause the pressure to which Ishiharada et al's device reacts.
Accordingly, it is an object of this invention to provide an optical switch which does not deteriorate through repetitious use, such as is the case with fiber optical switch devices.
Another object of this invention is to provide a device which is adapted to act directly on the light transmitting medium rather than on a secondary part of the structure as is the case in devices where a fiber optical member is surrounded by elastomeric coaxial components.
Yet another object of this invention is to provide an optical switch device which does not require the optically sensitive portion of the device to be linearly aligned during operation of the device.
Still another object of this invention is to provide an optical switch device which is protected in an enclosed space which itself provides boundaries for the force applied, so that a direct and proportional response to outside forces is supplied by the optical portion of the device.
Other objects will appear hereinafter.